An automatic transmission installed in a vehicle includes an input shaft to which driving force of a driving source such as an engine or a motor is transmitted, and a belt-type continuously variable transmission mechanism (CVT) installed between the input shaft and an output shaft. In the automatic transmission provided with such the continuously variable transmission mechanism, a desired gear ratio is obtained by shifting the rotation of the input shaft with the continuously variable transmission mechanism and transmitting it to the output shaft. And, such an automatic transmission includes a start clutch arranged between the output shaft of the continuously variable transmission and a rotating shaft (or a counter shaft) installed downstream thereof, for switching presence/absence of driving force transmission to the downstream side from the output shaft when a vehicle starts.
In a vehicle equipped with the above-described continuously variable transmission mechanism including the start clutch, judders (or vibrations) may occur in the start clutch due to the input of excessive torque to the continuously variable transmission mechanism from the driving source. Therefore, measures for suppressing the judders of the start clutch are necessary. On the other hand, it is also important to ensure the power performance of vehicle by allowing to transmit the torque of driving source to the drive wheels as effectively as possible. Therefore, it is desirable to attempt, by appropriately controlling the transmission torque of start clutch, to ensure both judder toughness (or anti-vibration performance) by suppressing the lifetime cumulative load of start clutch and power performance of the vehicle.
It is noted that as a conventional technology of transmission torque control of clutch related to the present application, for example, Patent Document 1 discloses a control device for reduction control of engine torque so that the transmission clutch for each gear position of an automatic transmission can not slip. This control device calculates torque limit values that the engine can output without slipping the transmission clutch (or the frictional engagement member) for each gear position of the automatic transmission, setting the engaging state of lockup clutch, the hydraulic oil temperature of automatic transmission, the engine rotational speed, and the shifting state of automatic transmission as parameters. This enables to minimize the reduction amount of engine torque, while certainly preventing slipping of the transmission clutch for each gear position.
Since the control device described in this Patent Document 1 performs control to prevent slipping of the transmission clutch in each gear position of a stepped automatic transmission, the control device has mapped slip prevention torque limit values of clutch for each gear position so as to restrict a requested torque when the said requested torque exceeds the said torque limit values. And, also in the engine torque control of a continuously variable transmission mechanism, while such control as in Patent Document 1 is already used for improving starting performance of product, setting oil temperature and clutch differential rotation as parameters, the problem of the present invention is to suppress the lifetime cumulative load of start clutch within a target, by appropriately controlling the input torque of start clutch accompanying the continuously variable transmission mechanism. Therefore, even if the control described in Patent Document 1 is applied as it is, since the load applied to the start clutch accompanying the continuously variable transmission mechanism cannot be controlled appropriately when the vehicle starts or during its subsequent running, it will not be a means to resolve the problem of the present invention.